1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a bonded disk, such as a laser disk and a bonded type digital video disk (DVD), by bonding two one-sided disks together.
2. Description of the Prior Art
Some optical disks, whose recorded information is read by using laser beams, are made by bonding together two disk substrates, one of which offers the first side of the finished disk and the other the second side. Examples include laser disks and DVDs. A laser disk is taken for example in describing the method of manufacturing such optical disks.
First, a die (called a stamper) formed beforehand with pits representing recorded information is mounted in a resin molding machine such as an injection molding machine. A transparent plastic resin such as a polycarbonate or acrylic resin is injection-molded or pressure-molded by the stamper to form a transparent plastic disk (one-sided disk) that is stamped with pits (substrate forming process). Following this, the surface of the disk formed with pits is deposited with a metal such as aluminum by sputtering or vacuum vapor deposition to form a reflection film (reflection film forming process). Then, the surface of the reflection film is coated with an ultraviolet-setting resin using a spinner or the like, and is irradiated with ultraviolet radiation (light) to form a protective film for protection against scores and corrosion (protective film coating process).
A pair of one-sided disks, which will form the A side and B side of a finished disk, are produced separately in this way. The surfaces of the one-sided which are to be bonded together are applied with a thermoplastic bonding agent referred to as hot melt using a roll coater or the like (bonding agent coating process). Then, the paired one-sided disks are stacked together so that the surfaces which are applied with the bonding agent contact each other, and then pressed together to form a single laser disk (bonding process).
In recent years, small digital video disks (DVDs) 12 cm in diameter capable of high-density, double-sided recording have been under development. In such small disks, a large part of the flat disk surface ranging from the center hole of the disk to the recording region is used as a clamp zone of the player. Hence, when the disk is placed on the turntable of the player and held by a clamper, the flat clamp zone works as a reference surface for the optical system. Further, in such a small, double-sided recording disk, there is no alternative but to use the flat clamp zone as a title display area of the disk.
When a title is printed, or a seal is "stuck," (viz., adhered in place) on the outer surface of the clamp zone of a small disk such as DVD, however, the following problem arises.
In a small, double-sided, recording disk such as DVD, the allowable range of disk tilt angle for reproduction is significantly small compared with that of the laser disk, so that even a slight tilt can degrade the reproduced signal substantially. When the clamp zone is used as the title display area, surface undulations caused by a seal or printed ink applied to the outer surface of the clamp zone result in an increased tilt of the disk.
To avoid this problem it is necessary that the title to be printed, or the seal to be stuck, on the inner bonding side of the disk, rather than on the outer side. When the title is displayed on the inner bonding surface, it is necessary to ensure that the printed ink and the seal do not affect the overall thickness of the bonded disk. For this purpose, some steps should be taken, such as cutting the title display area to a depth equal to the thickness of the printed ink or stuck seal (for example, 0.1 mm) to form a recess 1 as shown in FIG. 7A.
However, even this measure cannot solve the above problem because the thickness of the printed ink and seal may not always be constant. Particularly when letters are printed with ink, the ink 2 forms into bulges or projections, as shown in FIG. 7B. If a hot melt as the bonding agent is applied over these projections by the roll coater, the entire recess 1 in the title display area cannot be filled with the bonding agent, with the hot melt 3 coated only over the projections of the ink 2, as shown in FIG. 7C. Therefore, the area of the recess 1 is bonded only partially and therefore with a reduced bonding force.
In the case of the laser disk, the paired disk substrates to be bonded together are each 1.2 mm thick and have high rigidity (mechanical strength) so that the aforementioned deformation will not result. In the case of the DVD, however, each of the paired disk substrates measures 0.6 mm in thickness and does not have sufficient rigidity. Hence, when a thin disk such as DVD is stored or left at elevated temperatures for a long period, it is likely to develop deformations. Particularly when a disk is left mounted on the reproduction player for a long period, the disk held horizontal by the clamper is subjected to high temperatures due to internal heating of the player, so that the hot melt, the thermoplastic bonding agent, becomes soft allowing the disk outer periphery to droop under its own weight, making the whole disk look like a bowl turned upside down.
Further, the actual plastic disk (one-sided disk) formed during the substrate forming process has a ring groove d (hereinafter called a stamper clamp groove) for receiving the stamper clamp that fixes the stamper to the resin molding machine. The part of the disk where there is the stamper clamp groove has a smaller thickness than other areas and therefore a smaller strength. The reduced strength due to the stamper clamp groove d, combined with the recess 1 formed on the bonding inner surfaces of the one-sided disks, makes the bonded disk more likely to be deformed.
The present invention solves the above problem and its major objective is to provide a method and an apparatus for manufacturing a bonded disk, which can be mass-produced inexpensively, which prevents the bonding strength from being reduced even when the bonding inner surfaces are formed with a recess for title display, and which is not easily deformed even when it is stored or left at elevated temperatures for a long period.